Oil-paper, the relative order of the words corresponding to the preponderance of the oil with respect to paper in the insulation, is among the most widely used insulation systems after atmospheric air. Developed since the discovery of electricity, and thanks to its excellent dielectric, thermal, and arcquenching properties, the oil-paper insulation is used in high-voltage power apparatus, from power transformation equipment to circuit interrupters and breakers at various voltage levels. The oil-paper insulation has reached the level of 735 kV in the 1960s, 1000 kV in the 1990s, and touching the 1200 kV level at present. The main drawback resides in the high fire hazards associated with failure of oil-paper insulation, which has gradually removed the oil insulation from circuit interrupters and breakers and triggered several attempts to find a replacement to mineral oil with less flammable liquids. At present, the use of oil-paper insulation is limited to power equipment, mainly transformers and inductances, where the high energy losses required an effective cooling system that cannot be found with other insulation systems. In the oil-paper insulation, the main component is the mineral oil, which serves at the same time as the main insulating material and a cooling liquid. The cellulose paper, mainly in the form of pressboard, is also required mainly to provide a mechanical support to the high-voltage windings. This chapter reviews the various aspects of oil-paper insulation, namely, its physicochemical properties and the breakdown mechanisms active in this insulation system.